Many battery-powered hearing instruments or hearing devices provide access to the battery in order too allow replacement by a user. It may then happen that small batteries, especially disc-type batteries used in e.g. watches and in particular in hearing aids, are inserted the wrong way around mechanically. As a result, their electrical polarity with respect to the device's normal operating condition is inverted, that is, the positive terminal of the battery is connected to the negative power terminal of the device, and the negative terminal of the battery is connected to the positive power terminal of the device. The device will not operate and may even be damaged.
In order to overcome this problem, U.S. Pat. No. 5,623,550 proposes a rectifier circuit that accepts, at a pair of input terminals, a power supply whose polarity may be oriented either way, and provides, at a pair of output terminals, a voltage with a predefined polarity. In order to reduce losses—as compared to a diode rectifier bridge—the rectifier circuit comprises two transistor bridge circuits, with e.g. MOSFET transistors. The transistor switches are opened or closed in order to connect the input and output terminals according to the polarity at the input terminals.
The term “hearing instrument” or “hearing device”, as understood here, denotes on the one hand hearing aid devices that are therapeutic devices improving the hearing ability of individuals, primarily according to diagnostic results. Such hearing aid devices may be Outside-The-Ear hearing aid devices or In-The-Ear hearing aid devices. On the other hand, the term stands for devices which may improve the hearing of individuals with normal hearing e.g. in specific acoustical situations as in a very noisy environment or in concert halls, or which may even be used in context with remote communication or with audio listening for instance as provided by headphones.
The hearing devices as addressed by the present invention are so-called active hearing devices which comprise at the input side at least one acoustical to electrical converter, such as a microphone, at the output side at least one electrical to mechanical converter, such as a loudspeaker, and which further comprise a signal processing unit for processing signals according to the output signals of the acoustical to electrical converter and for generating output signals to the electrical input of the electrical to mechanical output converter. In general, the signal processing circuit may be an analog, digital or hybrid analog-digital circuit, and may be implemented with discrete electronic components, integrated circuits, or a combination of both.
Most hearing instruments have one or more manual switches. These can be used for instance to toggle programs or to increase/decrease the volume of the hearing instrument. In order to detect whether a switch has been pushed or not, the simple circuitry of FIG. 1 can be used, in which a first terminal of the switch is connected to the ground of the device, and a second terminal of the switch is connected, via a pull-up resistor, to the supply voltage of the device. The voltage at the second terminal of the switch indicates the state of the switch.
When used in a device comprising a rectifier circuit as described above, the circuit according to FIG. 1 can not be used, since it is not known whether the terminal that the switch is connected with shall carry—according to the way the battery is inserted—the ground or the supply voltage of the device. One way to solve this problem is to connect the first terminal of the switch to a ground terminal that is provided as an output of the rectifier, as shown in FIG. 2.
In many hearing instruments the integrated circuits (ICs) and the passive components like surface mounted device (SMD) resistors and capacitors are combined in a subassembly or electronic circuit assembly, also called a hybrid circuit. Such a hybrid circuit is combined with electromechanical components such as manual switches, and electro-acoustic components such as microphones and loudspeakers to constitute a hearing instrument.
It is desirable to minimise the number of input/output (IO) contacts that are needed to interface the hybrid circuit with the other components.
The same issues arise if the switch is replaced or supplemented by an analog input, such as a variable resistor for volume control, or by an active input device, such as a programming device that applies a bit sequence and transmission control signals to separate connections.